This invention relates to an activated sludge wastewater treatment process for removing phosphate from BOD-containing wastewater to obtain a substantially phosphate free effluent.
In the conventional activated sludge systems in use today, wastewater is subjected to the usual screening and pretreatment steps, e.g., primary sedimentation, then mixed with recycled activated sludge to form a mixed liquor which is subjected to aeration with an oxygen-containing gas in an aeration zone. During aeration of the mixed liquor, the microorganisms present in the activated sludge cause the aerobic decomposition of solids and a high degree of BOD removal is achieved.
Phosphates, which are present in organic wastes and detergents, escape conventional wasterwater treatment processes and are released with the effluent into natural water resources, e.g., lakes, rivers and streams. These phosphates result in over-fertilization or eutrophication of waters causing unsightly algal blooms and serious pollution problems.
It is known that aeration of the mixed liquor in an activated sludge wastewater treatment process initially causes the microorganisms present to take up phosphate. U.S. Pat. No. 3,236,766 discloses a process which utilizes this phenomenon for removing phosphates from wastewater. According to the process disclosed in that patent, the pH of the raw wastewater is adjusted, if necessary, to maintain a range of from about 6.2 to about 8.5, the wastewater is mixed with activated sludge to form a mixed liquor, the mixed liquor is aerated to maintain a dissolved oxygen content of at least 0.3 mg. per liter in the mixed liquor and a phosphate-enriched sludge is separated from the mixed liquor to provide a substantially phosphate-free effluent. The phosphate-enriched sludge is treated to reduce the phosphate content thereof prior to recycling for mixing with the influent wastewater. This is accomplished by maintaining the phosphate-enriched sludge in an anaerobic condition for several hours in a combination phosphate stripper and sludge thickener vessel. In this vessel, the phosphate-enriched sludge is settled and thickened and the anaerobic conditions cause the microorganisms which took up phosphate in the aeration zone to release phosphates to the liquid phase to form a phosphate-enriched supernatant. The phosphate-enriched supernatant is delivered to a phosphate precipitator where a phosphate precipitating reagent such as lime is added to precipitate the soluble phosphates.
In the practice of the above-described process, it has been found that a considerable time is required for the released soluble phosphate in the anaerobic, sludge, and particularly in the settled sludge in the lowermost section of the stripping zone, to migrate out of the settled sludge layer and into the supernatant liquor in the stripping zone. Such slow migration is a consequence of physical obstruction to the diffusional flow of released phosphate by the densely compacted sludge solids as well as inherent equilibrium limitations to the mass transfer process. Under such conditions, if the sludge is withdrawn from the stripping zone and recycled to the aeration zone before a sufficient amount of the soluble phosphate is transferred to the supernatant liquor, an excess amount of soluble phosphate is recycled to the aeration zone and the phosphate removal efficiency of the overall process is undesirably lowered. Thus, in practice, a considerable amount of soluble phosphate, of which the concentration increases from the top to the bottom of the stripper, is trapped in and carried out with the recycled sludge. In addition, the phosphate stripper must also function as a sludge thickener, and must, therefore, be designed for long retention times to accommodate such thickening function.
It has been proposed to increase the phosphate removal efficiency of the above-described system by various schemes including recirculating a portion of the supernatant liquor withdrawn from the stripping zone into the bottom of the stripper vessel, introducing the phosphate-enriched sludge into the bottom of the stripper vessel, or recirculating a portion of the sludge into the bottom of the stripper vessel, or recirculating a portion of the sludge withdrawn from the bottom of the stripper vessel to the supernatant liquor therein, so as to increase the level of soluble phosphate transfer to the supernatant liquor. Such modifications, while affording some improvement in the amount of soluble phosphate removed from the recycle sludge, are limited in their effectiveness by the fact that the stripping zone must function to thicken the recycle sludge despite the circulation and flows within the stripper vessel under these proposed schemes which tend to promote backmixing behavior and interfere with such thickening function.
Several other processes have been proposed for reducing the phosphate content of phosphate-enriched sludge following the aeration step in an activated sludge sewage treatment process. Thus, for example, U.S. Pat. No. 3,385,785 discloses adjusting the pH of phosphate-enriched sludge to between about 3.5 and 6 and agitating the sludge in contact with a low phosphate-containing aqueous medium in a tank for a time sufficient to effect transfer of water-soluble phosphate material from the sludge to the aqueous phase. After the soluble phosphate has been leached out of the sludge into the aqueous medium, the mixture is passed to a settling tank wherein the phosphate-enriched aqueous medium is separated from the phosphate-depleted sludge. The phosphate-enriched aqueous medium is then treated with a reagent such as lime to precipitate the soluble phosphate. Since this process requires the addition of considerable quantities of low phosphate-containing aqueous medium and required two separate tanks -- i.e., a phosphate leaching tank and a settling tank, to separate the soluble phosphate from the sludge, a large capital investment is required in such a process. In addition, large quantities of reagent may be required to maintain the pH of the elutriant aqueous medium within the prescribed limits and to precipitate soluble phosphate from the phosphate-enriched aqueous medium.
Various other systems have been proposed by the prior art which employ mixing of the phosphate enriched sludge with a lower phosphate aqueous medium followed by separation into phosphate-lower sludge, i.e., the treated recycle sludge, and phosphate enriched aqueous medium. Such systems are inherently limited by equilibrium effects since the soluble phosphate concentration in the liquid phase of the treated recycle sludge cannot be lower than the soluble phosphate concentration of the aqueous medium from which it is separated. Thus, improvement of the phosphate removal capability of this system requires either an increase in the total volume of the aqueous transfer medium or an increase in the number of mixing-separation steps, both costly solutions to the problem of achieving high phosphate removal efficiency.
Accordingly, it is an object of this invention to provide an improved process for reducing the phosphate content of phosphate-containing wastewater in an activated sludge wastewater treatment process.
It is a further object of this invention to provide such a process which includes an improved phosphate stripping of phosphorous-enriched sludge and which results in increased efficiency of the phosphate removal from the wastewater.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and claims.